1. Field of the Invention
The present invention relates to an electronically controlled timepiece that controls timepiece hand driving in response to a signal, as a reference, from an oscillator circuit that employs a time standard source such as a crystal oscillator, a power supply control method for the electronically controlled timepiece and a time correction method for the electronically controlled timepiece.
2. Description of the Related Art
In one of known electronically controlled mechanical timepieces that are controlled by making use of an IC or a crystal oscillator, a generator converts, into electrical energy, mechanical energy released by a mainspring, the electrical energy drives a rotation controller, which controls a current flowing through a coil of the generator, and hands secured to train wheels that transmit the mechanical energy from the mainspring to the generator are accurately driven to indicate accurate time.
Electrical energy from the generator is once stored in a smoothing capacitor, and the power from the capacitor drives the rotation controller. Since the capacitor is supplied with an alternating-current electromotive force in synchronization with the rotation period of the generator, it is not necessary to store power for a long period of time to enable the rotation controller having an IC or a crystal oscillator to operate. Conventionally, a relatively small capacitance capacitor enabling the IC or the crystal oscillator to operate for several seconds, i.e., a capacitor of 10 μF or so is employed.
The electronically controlled mechanical timepiece needs no motor because the mainspring is a power source for driving timepiece hands, and is low cost with a small component count. It is sufficient if a small amount of electrical energy needed to drive an electrical circuit is generated. A small input energy is enough to drive the timepiece.
The electronically controlled mechanical timepiece has the following drawback. When a time correction operation (a timepiece hand setting operation) is performed with the crown pulled out, each of an hour hand, a minute hand, and a second hand is stopped to set an accurate time. The stop of the hands stops train wheels, and thus the generator as well.
The input of the electromotive force to the smoothing capacitor from the generator is suspended, while the IC is continuously driven. The charge stored in the capacitor is discharged to the IC side, and a voltage across terminals of the IC gradually drops. The voltage applied to the IC thus drops below an oscillation stop voltage (Vstop, for instance, 0.6 V), leading to the stop of the rotation controller.
When the oscillation of the IC stops, the power consumption is reduced, and the voltage drop rate in the capacitor also becomes slow. When the time correction operation takes time long enough to cause the voltage of the capacitor to drop below the oscillation stop voltage, the capacitor typically falls to a voltage of 0.3 to 0.4 V slightly lower than the oscillation stop voltage. When the time correction operation (hand setting time) becomes excessively long, to several minutes, for instance, the capacitor is fully discharged with the voltage thereof dropped to zero V.
Even if the generator starts rotating with the crown pushed into after the hand setting, the capacitor, the voltage of which has once dropped below the oscillation stop voltage as a result of discharge, takes time before the capacitor is charged again to be high enough to reach a drive start voltage (voltage capable of driving the IC) for the rotation controller. The IC (an oscillator circuit) remains inoperative throughout, and no accurate time control is performed.
Specifically, when the crown is pulled out to a second step (for a hand setting mode) from a zero step (for a normal hand driving mode) or from a first step (for a calendar correction mode) at time point A as shown in FIG. 26, the rotor of the generator stops, stopping charging a capacitor C1. On the other hand, the capacitor C1 continuously feeds electrical energy to the rotation controller (including a “drive IC” in a drive circuit for driving the crystal oscillator as a time standard source), thereby allowing the crystal oscillator to continuously oscillate.
The voltage of the power source capacitor C1 gradually drops. At time point B1 (within three minutes from time A, for instance), the hand setting operation ends, and the crown is pushed in, moving from the second step to the first step or zero step (for the normal operation). The generator becomes operative again, restarting the charging of the power source capacitor C1, and raising the voltage of the power source capacitor C1. In this case, the oscillation of the crystal oscillator continuously oscillates, the drive circuit (the rotation controller) quickly resumes rotation control of the rotor (brake control), and an indication error subsequent to the hand setting becomes zero.
When the hand setting operation is prolonged to be longer than three minutes, for instance, the voltage of the capacitor C1 drops below the oscillation stop voltage (Vstop, 0.6 V, for instance) of the drive circuit, and the oscillation stops at time B2 at the moment the hand setting operation ends. Even if the crown is moved to the first step at point B2, the rotation controller takes the sum of time T1 and time T2 before it resumes rotation control of the rotor, leading to an indication error.
The time T1 is a duration of time, during which the power source capacitor C1 is charged to a voltage (Vstart) on which the drive circuit and the oscillator circuit in the rotation controller normally operate. The voltage Vstart is typically higher than the voltage Vstop, and is 0.7 V, for instance.
The time T2 is a duration of time from the application of the oscillation start voltage (Vstart) until the oscillator circuit starts oscillating. The time T2 becomes longer as the voltage of the power source capacitor C1 is lower, and ranges from several seconds to several minutes, as shown in FIG. 27. For instance, when the oscillation start voltage (Vstart=0.7 V) is reached with the power source capacitor C1 gradually charged, the time T2 is approximately 20 seconds with the voltage (0.7 V) applied thereto.
When the hand setting operation takes time, the voltage of the power source capacitor C1 drops, thereby stopping the oscillation. Subsequent to the end of the hand setting operation, the oscillator circuit takes time T1+T2 before the start of the oscillation. Because of a lower voltage applied thereto, the oscillator circuit takes several seconds to several minutes for T2 alone. Before the start of the oscillation, the rotation of the rotor is not controlled. The hands gain or lose time, suffering from a substantial indication error.
The use of a large capacitance capacitor C1 to permit a longer hand setting time is contemplated. The oscillator circuit is thus prevented from stopping even if the hand setting takes three minutes or longer.
The use of a large capacitance capacitor slows the rise rate of the power source voltage. When the mainspring is released and stopped, it takes a long time to increase the voltage across the capacitor from the state in which no charge is stored in the power source capacitor. For a long time from the start of tightening of the mainspring to the rise of the power source voltage, the hands remain unable to present accurate time. In this case, there is a possibility that the user may mistake the state for a timepiece failure. Increasing the capacitance of the capacitor is thus not practical.
Increasing the power generation capacity of the generator to complete charging in a short time is contemplated. This arrangement increases the size of the generator, and also needs to increase the size of the mainspring as the torque to be transferred from the mainspring for feeding mechanical energy to the generator increases. This arrangement cannot be adopted for use in wristwatches, which are subject to the limitation of area and thickness dimensions.
In some of a variety of electronically controlled timepieces, such as a self-winding generator timepiece, a solar-cell charging timepiece, a battery driven timepiece, other than the electronically controlled mechanical timepiece, an oscillator circuit or an IC is stopped during a time correction operation to reduce power consumption and to prolong operation time. In this case, it takes several seconds to several minutes for the oscillator circuit to stably operate. A time error is also introduced.
It is an object of the present invention to provide an electronically controlled timepiece, a power supply control method for the electronically controlled timepiece, and a time correction method for the electronically controlled timepiece.